YK-11: How It Differs from SARMs, the Follistatin Mechanism and State of Evidence

YK-11 is sometimes grouped with RAD-140 and LGD-4033 as another SARM. That comparison is misleading on two levels, structural and mechanistic, and this distinction is the central theme of this article. It matters because the evidence base for YK-11 is even thinner than for other compounds in this category.

This article reviews the structure, mechanism and state of the evidence. It does not provide doses or protocols for use.

All substances discussed in this article are research compounds and are not intended for human consumption. Their effects are described solely on the basis of laboratory studies and experimental observations, not clinical evidence.

How YK-11 differs from RAD-140 and LGD-4033

First difference: structure

The SARM class is usually defined by two features: selective activity at the androgen receptor and a non-steroidal structure. The second feature distinguishes SARMs from testosterone and its derivatives.

YK-11 does not meet this structural criterion. Its full chemical name, methyl (17alpha,20E)-17,20-[(1-methoxyethylidene)bis(oxy)]-3-oxo-19-norpregna-4,20-diene-21-carboxylate, indicates a steroidal scaffold derived from 19-norpregnadiene. Structurally, it is therefore a steroidal compound, not a non-steroidal modulator such as RAD-140 or LGD-4033.

Second difference: mechanism

This is the most distinctive and best-documented aspect of YK-11. A study in C2C12 myoblasts showed that YK-11 induced myogenic differentiation while acting as a partial androgen receptor agonist that activates the receptor without N/C interaction (Kanno et al., 2013, PMID 23995658).

More importantly, the authors showed that YK-11, unlike dihydrotestosterone, induced follistatin expression, and that an anti-follistatin antibody abolished the resulting myogenic differentiation. This indicates that the anabolic effect observed in this model was mediated to a substantial extent by follistatin rather than androgen receptor stimulation alone. Induction of the myogenic regulatory factors MyoD, Myf5 and myogenin was more pronounced than with dihydrotestosterone.

Because follistatin binds and neutralizes myostatin, a protein that limits muscle growth, YK-11 is sometimes described in the literature as a myostatin inhibitor (Lee et al., 2021, PMID 33588136). This is shorthand: the compound does not inhibit myostatin directly but increases the expression of its natural antagonist.

How should it be classified?

The literature is inconsistent: the same molecule is described both as a SARM and as a myostatin inhibitor. The most precise description is that YK-11 is a steroidal partial androgen receptor agonist with an additional follistatin-dependent anabolic mechanism. Placing it in the same group as non-steroidal SARMs obscures both distinctions. The broader classification is explained in our overview of SARMs as a group.

State of the evidence: the weakest of the three

No human clinical trials of YK-11 have been published. There are no human data on efficacy, pharmacokinetics or safety from either registration trials or early-phase studies.

The available literature is divided into three groups:

• Cell-culture studies – primarily the C2C12 myoblast model described above (PMID 23995658).
• Animal-model studies – including research on myostatin in mouse skeletal muscle under conditions of infection (PMID 33588136).
• Analytical studies for anti-doping control – an in vivo metabolism study identifying metabolites useful in doping control (Piper et al., 2018, PMID 30379415), and a corresponding study in an equine model (Harding et al., 2023, PMID 36519889).

The proportions are revealing: more is known about detecting YK-11 in anti-doping control than about its effects in humans.

Central nervous system observations

A 2024 study evaluated the effects of YK-11 on the rat hippocampus using pharmacokinetic modeling, molecular docking and in vivo and ex vivo experiments (Dahleh et al., 2024, PMID 38521455). Two findings warrant attention:

• Substantial brain penetration was indicated by pharmacokinetic modeling, suggesting that the compound’s effects are not necessarily confined to skeletal muscle.
• Inhibition of type II 5-alpha-reductase was indicated by molecular docking, suggesting an additional mechanism beyond androgen signalling in muscle.

These findings come from an animal model and computational analyses, not clinical data. They nevertheless indicate that the compound’s activity may extend beyond the tissue highlighted in promotional claims.

Regulatory status

YK-11 is not registered as a medicinal product in any jurisdiction. It is not a dietary supplement, food or cosmetic and is not eligible for authorized health claims. No authorized body has established recommendations for its use in humans.

Systematic reviews of SARM safety in healthy adults (Vignali et al., 2023, PMID 37218811) and SARM use by athletes (Vasireddi et al., 2025, PMID 39755947) can address YK-11 only to the limited extent permitted by the sparse evidence described above.

Anti-doping note

YK-11 falls within category S1.2 of the WADA Prohibited List: other anabolic agents. It is prohibited at all times, both in and out of competition. Metabolites useful for anti-doping control have been identified and characterized (PMID 30379415), and analytical methods for detecting compounds in this group are described in the literature (Thevis and Schänzer, 2018, PMID 28137616).

YK-11’s steroidal structure does not alter its classification under S1.2, and it may also be assessed in the context of steroid-profile parameters. In Poland, the rules are administered by the Polish Anti-Doping Agency (POLADA), and amateur athletes participating in covered competitions may also be tested. Because YK-11 is not a registered medicine, there is no basis for a Therapeutic Use Exemption for this compound.

What is actually in the product?

An analysis published in JAMA examined 44 products sold online as SARMs. Only 52% contained any SARM, 39% contained another unapproved drug, and 9% contained no active compound (Van Wagoner et al., 2017, PMID 29183075). A European network of official laboratories also examined products of suspected illegal origin for SARMs, metabolic modulators and growth hormone secretagogues (Barrios et al., 2025, PMID 40551438).

For a compound as sparsely characterized as YK-11, this issue is especially important: identity and purity can be assessed only through batch-specific analytical documentation, meaning a set of analyses that confirms identity and purity rather than a name printed on the label.

Harm reduction

• A human safety profile has not been established. It cannot be characterized as favorable or unfavourable because no human studies have been conducted.
• The steroidal structure matters. The claim that YK-11 is ‘not a steroid, but a SARM’ is structurally incorrect: YK-11 has a steroidal scaffold.
• Its activity may not be limited to muscle. Animal and computational models indicate substantial brain penetration and inhibition of type II 5-alpha-reductase.
• Class-wide risks remain relevant. These include suppression of the hypothalamic-pituitary-gonadal axis and liver injury reported with compounds in this group, as discussed in the parent article.
• Combining substances multiplies the unknowns. A poisoning case following concurrent exposure to two compounds in this area has been reported (Kintz et al., 2021, PMID 34678947).

Frequently Asked Questions

Is YK-11 a SARM?

The literature is inconsistent, but two points are decisive. Structurally, YK-11 is a steroidal compound, whereas classic SARMs such as RAD-140 and LGD-4033 are non-steroidal. Mechanistically, it is a partial androgen receptor agonist whose anabolic effect in the experimental model was mediated largely by follistatin. The most precise description is ‘a steroidal partial androgen receptor agonist with a follistatin-dependent mechanism’.

Does YK-11 inhibit myostatin?

Not directly. YK-11 induces follistatin expression, and follistatin binds and neutralizes myostatin. In C2C12 myoblasts, an anti-follistatin antibody abolished the myogenic effect (PMID 23995658).

Are there YK-11 studies in humans?

No. No clinical studies assessing the efficacy or safety of YK-11 in humans have been published. Available evidence comes from cell cultures, animal models and analytical work for anti-doping control.

Does YK-11 only work on muscles?

There is no basis for that claim. A 2024 study indicated substantial brain penetration and, in a molecular docking analysis, inhibition of type II 5-alpha-reductase (PMID 38521455).

Why does the article not give doses?

Because YK-11 is not a medicinal product and has never undergone human dose-finding studies. Values circulated online do not come from clinical trials.

Summary

YK-11 is often grouped with SARMs, but comparison with RAD-140 and LGD-4033 obscures two important differences. Structurally, YK-11 is a steroidal 19-norpregnadiene derivative, whereas classic SARMs are non-steroidal. Mechanistically, it is a partial androgen receptor agonist that activates the receptor without N/C interaction, while its anabolic effect in C2C12 myoblasts was mediated by follistatin. Follistatin neutralizes myostatin, so YK-11 is sometimes called a myostatin inhibitor even though it does not inhibit myostatin directly. The evidence base is particularly weak: no human studies have been published, and available data come from cell cultures, animal models and anti-doping analysis. A 2024 study also indicated substantial brain penetration and inhibition of type II 5-alpha-reductase. YK-11 is classified under WADA category S1.2 and is prohibited at all times.

Product documentation

Research material pages with batch-specific analytical documentation are available for the capsule form and the 30 ml solution. The complete range is listed under SARMs and modulators. These materials are intended for research purposes only.

Bibliography

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